WASHINGTON
— As Washington continues to rev up investigations into whether the US government could have stopped the Sept. 11 attacks, one agency is getting ever more scrutiny: the Federal Bureau of Investigation.

It was within the FBI that two of the most glaring apparent lapses occurred: failing to follow up on an internal tip about Arab men training at US flight schools and not authorizing a search of the computer of a Minnesota man suspected of terrorist ties.

Evidence of FBI missteps is generating political pressure to reshape the crime-busting bureau into a kind of domestic terror shield  or create a new domestic intelligence agency to take on that role.

"It's almost as if the entire agency has to be restructured from the bottom up  and my suspicion is that's what Congress will demand," says Marshall Wittmann, a congressional analyst at the Hudson Institute here.

Already, the pressure has led to some reforms. Director Robert Mueller is expanding the FBI's analysis divisions, importing more than 25 CIA agents to help guide reforms, and buying banks of new computers. Observers say he's also trying to bust up a culture of caution and turf-guarding.

But the bureau's history may be working against him.

That history includes rivalry with the CIA, bungled cases, and a legacy of solving  more than preventing  crimes.

Also, in contrast to the CIA, the bureau's domestic beat has made it wary of infringing civil liberties in its efforts to hunt down criminals. Warrants to search computers, for example, were rarely granted before Sept. 11  as the FBI sought to avoid abusing its powers.

The effort to search the computer hard drive of so-called "20th hijacker" Zacarias Moussaoui was detailed in a blistering memo from Coleen Rowley, the FBI's general counsel in Minneapolis, accusing headquarters of blocking a key terrorism investigation before Sept. 11  and concealing its efforts afterward.

"I have deep concerns that a delicate and subtle shading/skewing of facts by you and others at the highest levels of F.B.I. management has occurred and is occurring," Ms. Rowley wrote to Director Mueller, according to a copy obtained by Time magazine.

Now, congressional intelligence committees  and possibly a broader blue-ribbon panel  are set to launch full-scale investigations and are demanding change at the FBI.

"I just think they've got to go through a big learning curve, a lot of readjustment," said House Intelligence Committee Chairman Porter Goss (R) of Florida on "Fox News Sunday."

But criticism and calls for reform are hardly new.

Storied, and troubled, past

From the days of J. Edgar Hoover  the FBI director who kept secret files on Martin Luther King Jr., John F. Kennedy, and many others  through the 1990s, the FBI has long been criticized.

To be fair, it also has a storied history of tracking down mafia bosses, bank robbers, and other tough criminals.

But the 1990s were a tough decade, including such debacles as the destruction of the Branch Davidian compound at Waco, Texas, and the botched prosecution of scientist Wen Ho Lee.

In fact, Ronald Kessler, author of "The Bureau: The Secret History of the FBI," says Louis Freeh  the director for much of the 1990s  created "a sense of political correctness and risk-averseness that drove out many of the best managers and discouraged people from telling the truth."

Now there's huge momentum for reform  although some critics doubt the FBI can ever do good counterterrorism work and dangle the prospect of a new domestic intelligence agency.

That prospect raises one of the fundamental issues the FBI has long struggled with  the traditional American wariness of giving domestic law enforcement too much power.

In fact, concern about intruding on cherished liberties is the reason for the traditional wall of separation between the CIA  which has broad intelligence gathering powers  and the FBI, which doesn't. It's also the source of the long-simmering CIA-FBI tensions.

The memo also asserts that top FBI officials  possibly including Mueller  have since tried to obscure any sense that the bureau had advance warnings of the attacks. Headquarters "decided to circle the wagons," she writes, "in an apparent effort to protect the FBI from embarrassment and the relevant FBI officials from scrutiny."

But now the scrutiny is growing. The Senate intelligence committee plans 9/11 hearings starting June 4. And the White House and Congress are still wrangling over whether a blue-ribbon panel will hold more-public hearings on the topic.

On Sunday, Senate majority leader Tom Daschle (D) of South Dakota continued to press for an independent commission to investigate pre-9/11 intelligence failures.

The Bush administration has said it believes the proper channels for investigation are Congress's intelligence committees  which can keep classified information secret.